Yeah, always forget this forum doesn't have it in the default info. It's an '01 4.4. The smaller wheel bolted up fine, no clearance issues brake or otherwise and the lugs were fine. Vehicle drove normal with no DSC issues an no lights, although I didn't take it on the highway. I agree - doesn't seem like a good idea but it actually worked just fine.

Turns out it was all a moot point though. I didn't realize it at the time, but they didn't add the child seat latch system until '03 so we just took the wife's Mercedes.

Can't believe the debate this sparked...

As stated, I wanted more space, although the X barely has more than the w211 sedan with a half rear seat. The '01 X5 also doesn't have the child safety latch system, so the debate on tire size ended up a moot one and I did just take another car. Anyway I agree, it's not just about wheel size, it was others who were hung up on that rather than overall diameter. For all the naysaying, I tried it and it worked just fine - no clearnace issues, no lights, no DSC problems. It's surely not ideal, but it'll work fine in a pinch. I was also well beyond verifying bolt pattern and fitment issues when asking the initial question, although I suppose I didn't make that clear. Plenty of cars have compact spares and they don't ruin their diff by using them. The AWD/TC was really the compounding factor.

Whoever recommended AAA...they suck as far as I'm concerned, and sitting on the side of the road on Christmas day waiting on a tow was out of the question.

No, that was not me.

Issue #1 - Make sure the wheel fits the hub and bolt circle.
Issue #2 - Make sure the outside tire diameter is as close as possible to the one it is replacing (plus or minus one inch is the range to stay in)
Issue #3 - Turn off the DSC / Traction Control

All three differentials are "open" differentials. That is they have no self-contained lockup mechanism or limited slip clutches to worry about. A slight mismatch in diameter won't be a problem if speeds are kept at 35-40 mph.

2002 X5 3.0 245,300 miles
2004 325i 109,000 miles